Carbon dioxide (CO2) emissions are of current concern for power generation and for industrial sectors such as cement and steel making. Oxy-fuel combustion is considered as one of the promising technologies to capture CO2 from power plants. In oxy-fuel combustion, the fuel is typically burned in an O2/CO2 environment. Successful implementation of this technology necessitates a thorough understanding of the method and operation of combustion devices for solid fuels in O2 environments as well as an understanding of the influence of O2/CO2 mixtures with different oxygen concentrations.
The burner stability, NOx emission, char burnout, heat transfer to reactor walls, or gas temperature profiles have been reported for specific oxy-fuel combustion configurations. To match the temperature profile of air combustion, the oxygen level in these prior art oxy-combustion configurations have been limited, usually to less than 30%. Replacement of N2 with CO2 or recycled flue gas leads to changes in solid fuel particle devolatilization, ignition, and flame shape. These changes lead, in turn, to changes in furnace heat transfer, NOx emission rates, and char burnout. Studies have showed the influence of increasing oxygen concentration on devolatilization and ignition and concluded that, at higher oxygen concentration, devolatilization and ignition generally occur more rapidly.
NOx reduction is a potential advantage of oxy-fuel combustion. Oxy-fuel combustion with recycled flue gas can in some circumstances reduce NOx by about one third to one half compared to air combustion. For pulverized coal combustion in air, the total NOx is divided between about 20% thermal NOx, 80-100% fuel NOx, and minimal prompt NOx.
The extent of combustion of coal char is of significant concern in the design of a coal-fired combustion system. Researchers have studied ignition and devolatilization of coal particles in O2/CO2 conditions and compared them with air-fired conditions. Char burnout at 21% O2 in CO2 takes place at a slower rate than in air. However, at 30% O2 in CO2 the burnout is similar and at higher O2 concentrations, the char burnout proceeds much faster due to higher partial pressures of oxygen surrounding the char. Nevertheless, under O2/CO2 combustion conditions, an increase in the amount of oxygen relative to CO2 necessarily requires a decrease in the amount of the CO2. One of ordinary skill in the art of oxy-combustion of solid particulate fuels will readily recognize that a decrease in the amount of CO2 injected as an oxidant stream will result in decreased mixing between the CO2-containing oxidant stream and the solid fuel particles and that such decreased mixing will tend to outweigh any benefits associated with localized areas of higher oxygen assisting in burnout. Additionally, one of ordinary skill in the art will similarly recognize that an increase in oxygen level may bring about an increase in fuel NOx.
Some researchers have investigated various aspects of oxy-combustion of solid particulate fuels but little knowledge exists on the fine balance that needs to be drawn between adding too much or too little oxygen or too much or too little CO2 while still achieving good burnout and avoiding the formation of fuel NOx.
Nikzat et al. described a burner with high oxygen partial pressure. “Characteristics of Pulverized Coal Burner Using a High-oxygen Partial Pressure”, Chemical Engineering Research and Design, 82 (A1):99-104. That research focused on the stable combustion intensity and fundamental characteristics of the flame for different stoichiometric ratios. While the results show good burnout and low NOx emissions, the operating conditions and oxygen distribution scheme (if any) are not described.
Each of WO 2011/051463 and WO 2011/051464 disclose a solid fuel burner and a method to combust solid particles.
Still others have addressed burnout and NOx emissions for oxy-coal combustion under relatively low O2 concentration conditions. Relatively low O2 concentrations conceivably would produce less fuel NOx but the flame temperature is lowered by the dilution with the non-oxygen components in the carrier gas and oxidant streams. This is a significant disadvantage when a relatively hot flame is needed for greater heat transfer, especially in industrial melting furnaces or cement kilns.
In summary, there is relatively little knowledge regarding the impact upon burnout and NOx emissions for oxy-coal combustion under relatively higher O2 concentrations. Thus, there is limited understanding of the necessary operating conditions for such combustion methods where good burnout and low NOx emissions are achieved.
Thus, it is an object of the invention to provide a method of oxy-combustion of a particulate solid fuel that produces a relatively hot flame and avoids the problem of undesirably high levels of fuel NOx. It is another object of the invention to provide a method of oxy-combustion of a particulate solid fuel that produces a relatively hot flame and avoids the problem of poor burnout.